The present invention relates to a process for preparing a fluorine-containing elastomeric copolymer composition comprising a fluorine-containing elastomeric copolymer (hereinafter referred merely to as a xe2x80x9cfluororubberxe2x80x9d) and, more particularly, to a process for preparing a fluorine-containing elastomeric copolymer composition containing a polyhydroxy compound or a salt thereof as a vulcanization component.
Fluororubbers obtained from vinylidene fluoride and at least one other fluorine-containing monomer have been used in various industries because of excellent heat resistance, chemical resistance and oil resistance. It is necessary to cure a polymer by vulcanization so as to satisfy the properties described above under severe conditions.
The fluororubber is prepared by copolymerizing a monomer using a water-soluble initiator and/or an oil-soluble initiator. For example, Japanese Kokoku (Examined) Patent Publication No. 56-29895 and Japanese Kokoku (Examined) Patent Publication No. 56-34009 (corresponding to U.S. Pat. No. 4,141,874) disclose a process for preparing a fluororubber by a two-stage polymerization, wherein a water-soluble initiator is used in a first stage of the polymerization and an oil-soluble initiator is used in a second stage of the polymerization. In the second stage of the polymerization using the oil-soluble initiator, the oil-soluble initiator is homogeneously dispersed by adding an emulsifier to a medium.
The resulting fluororubber is mixed with a vulcanizing agent (for example, a polyhydroxy compound) and a vulcanization accelerator (for example, a quaternary ammonium salt) to give a composition for vulcanization.
The process for mixing the fluororubber with the vulcanizing agent and the vulcanization accelerator is roughly classified into a process for coagulating an aqueous dispersion (of a fluororubber by adding a vulcanizing agent and a vulcanization accelerator to the aqueous dispersion, and a process of isolating a fluororubber from an aqueous dispersion of fluororubber by coagulation, drying the fluororubber and kneading the fluororubber with a vulcanizing agent and a vulcanization accelerator.
Japanese Kokoku (Examined) Patent Publication No. 56-34009 (corresponding to U.S. Pat. No. 4,141,874) discloses a fluororubber composition obtained by mixing a fluororubber with an acid acceptor, an aromatic polyhydroxy compound and a specific quaternary ammonium compound and kneading the mixture.
However, the kneading operability is poor and it is difficult to homogeneously mix the fluororubber with the vulcanizing agent and the vulcanization accelerator. Furthermore, the kneading of the fluororubber having high viscosity requires large energy. The quaternary ammonium compound adsorbs moisture in an atmospheric air because of its hygroscopicity and, therefore, it is inferior in handling properties.
Japanese Kokai (Unexamined) Patent Publication No. 56-147840 discloses a process for preparing a composition for vulcanization of a fluororubber, which comprises adding a quaternary ammonium compound and/or a quaternary phosphonium compound, as a vulcanization accelerator, to an aqueous emulsion or an aqueous dispersion of a fluororubber, thereby to cause coagulation. According to this process, the fluororubber, the vulcanizing agent and the vulcanization accelerator can be homogeneously mixed and the efficiency is better than that in case of kneading.
However, because of its hygroscopicity, the quaternary ammonium compound tends to be incorporated into the aqueous medium during the coagulation from the aqueous dispersion, together with the fluororubber. Therefore, when a predetermined amount of the quaternary ammonium compound is intended to be incorporated into the fluororubber, at least a predetermined amount of the quaternary ammonium compound must be added to the aqueous medium and the amount of it to be incorporated into the fluororubber is changeable.
Both of the aqueous fluororubber dispersion and the aqueous fluororubber emulsion obtained using an oil-soluble initiator contain an emulsifier. However, it is known that the action of the quaternary ammonium compound is inhibited by the emulsifier.
WO96/17876(PCT/JP95/2485) describes that a polyhydroxy aromatic compound and a quaternary phosphonium salt are incorporated into the fluororubber prepared by a two-stage polymerization described above by kneading. However, it is known that the action of the quaternary phosphonium salt is also inhibited by the emulsifier.
Japanese Kokoku (Examined) Patent Publication No. 57-14775 discloses that a fluororubber is kneaded with a solid solution of a quaternary ammonium compound and a polyhydroxy aromatic compound in order to avoid an influence of the hygroscopicity of the quaternary ammonium compound. However, according to this process, poor dispersion of the solid solution occurs and thus vulcanization physical properties of the molded article are not stable.
Japanese Kokai (Unexamined) Patent Publication No. 50-22048 discloses a process for preparing a fluorine-containing elastomeric copolymer composition, which comprises continuously feeding a wetted fluorine-containing elastomeric copolymer bulk having a water content of about 15 to 35% by weight, which is a coagulate of a fluorine-containing elastomeric copolymer latex, to a mixing region, feeding a vulcanizing agent and a vulcanization accelerator in the form of fine powders, and mixing them while maintaining the particle size of the fluorine-containing elastomeric copolymer bulk. The publication describes with respect to this mixing operation that the vulcanizing agent and the vulcanization accelerator should be fed to the mixing region in the form of fine powders in an aqueous dispersion and that the temperature should be maintained at low temperature enough to prevent the adverse reaction between the fluorine-containing elastomeric copolymer and the curing components or other additives, which are present in the mixture. However, the quaternary phosphonium salt as the curing component is easily dissolved because of its water solubility, so that the quaternary phosphonium salt is consumed together with separated water when treated in an apparatus such as extruder. Since the mixing operation is conducted while maintaining the particle size of the fluorine-containing elastomeric copolymer bulk, there is a limit in homogeneity of the composition. Furthermore, it is difficult to continuously feed a constant amount of a fine-powdery solid material and, therefore, a predetermined amount of it cannot be incorporated and vulcanization physical properties of the molded article are not stable.
The fluorine-containing elastomeric copolymer composition is generally prepared by kneading a fluororubber with a vulcanizing agent and a vulcanization accelerator using an apparatus such as kneader. However, the kneading operation in a kneader requires large energy and charge of raw materials into the kneader is hard for an operator. The vulcanizing agent and the vulcanization accelerator are usually in the form of powders and, therefore, risk to the operator in case of charging them into the kneader is considered to be a serious problem.
An object of the present invention is to provide a process for preparing a composition for vulcanization of a fluororubber in a good efficiency.
Another object of the present invention is to provide a process which makes it possible to obtain a fluororubber composition having stable vulcanization physical properties, wherein a vulcanizing agent and a vulcanization accelerator are homogeneously dispersed in a fluororubber.
The present invention provides a process for preparing a fluorine-containing elastomeric copolymer composition comprising a fluorine-containing elastomeric copolymer, a vulcanizing agent and a vulcanization accelerator,
said process comprising the step of coagulating the copolymer from an aqueous dispersion of the fluorine-containing elastomeric copolymer and drying the resulting coagulate of the copolymer while kneading, wherein
a mixture of mutually soluble components consisting of the vulcanizing agent and the vulcanization accelerator is added to the coagulate.
The fluororubber is an elastomeric copolymer of vinylidene fluoride and at least one other fluorine-containing monomer. Examples of the other fluorine-containing monomer include tetrafluoroethylene, trifluorochloroethylene, trifluoroethylene, hexafluoropropylene, pentafluoropropylene, perfluoro(methyl vinyl ether), perfluoro (ethyl vinyl ether), and perfluoro (propyl vinyl ether).
The aqueous dispersion of the fluororubber may be prepared by any of polymerization processes known in the prior arts. The process of the present invention can be applied to an aqueous dispersion obtained by a two-stage polymerization process wherein a water-soluble initiator is used in a first stage of the polymerization and an oil-soluble initiator is used in a second stage of the polymerization, which aqueous dispersion may contain an emulsifier.
The vulcanizing agent includes, for example, a polyhydroxy compound. The polyhydroxy compound used as the vulcanizing agent is preferably a polyhydroxy aromatic compound. As the polyhydroxy aromatic compound, any of compounds known as the vulcanizing agent of the fluororubber can be used. Preferred examples thereof include 2,2-bis(4-hydroxyphenyl)propane (bisphenol-A), 2,2-bis(4-hydroxyphenyl)perfluoropropane (bisphenol-AF), resorcin, 1,3,5-trihydroxybenzene, 1,7-dihydroxynaphthalene, 2,7-dihydroxynaphthalene, 1,6-dihydroxynaphthalene, 4,4xe2x80x2-dihydroxydiphenyl, 4,4xe2x80x2-dihydroxystilbene, 2,6-dihydroxyanthracene, hydroquinone, catechol, 2,2-bis(4-hydroxyphenyl)butane (bisphenol-B), 4,4-bis(4-hydroxyphenyl) valeric acid, 2,2-bis(4-hydroxyphenyl)tetrafluorodichloropropane, 4,4xe2x80x2-dihydroxydiphenylsulfone, 4,4xe2x80x2-dihydroxydiphenylketone, tri(4-hydroxyphenyl)methane, 3,3xe2x80x2,5,5xe2x80x2-tetrachlorobisphenol-A, and 3,3xe2x80x2,5,5xe2x80x2-tetrabromobisphenol-A. Alkali metal salts or alkali earth metal salts thereof can also be used.
The vulcanization accelerator includes, for example, an onium compound. Examples of the onium compound include an ammonium compound, a phosphonium compound, an oxonium compound, and a sulfonium compound. A quaternary ammonium salt and a quaternary phosphonium salt are preferred.
As the quaternary ammonium salt or quaternary phosphonium salt, any of those known as the vulcanization accelerator of the fluororubber can be used. Preferred examples thereof include: quaternary ammonium salts, for example, 8-methyl-1,8-diazabicyclo[5.4.0]-7-undecenium chloride, 8-methyl-1,8-diazabicyclo[5.4.0]-7-undecenium iodide, 8-methyl-1,8-diazabicyclo[5.4.0]-7-undecenium hydroxide, 8-methyl-1,8-diazabicyclo[5.4.0]-7-undecenium methyl sufate, 8-ethyl-1,8-diazabicyclo[5.4.0]-7-undecenium bromide, 8-propyl-1,8-diazabicyclo[5.4.0]-7-undecenium bromide, 8-dodecyl-1,8-diazabicyclo[5.4.0]-7-undecenium chloride, 8-dodecyl-1,8-diazabicyclo[5.4.0]-7-undecenium hydroxide, 8-eicosyl-1,8-diazabicyclo[5.4.0]-7-undecenium chloride, 8-tetracosyl-1,8-diazabicyclo[5.4.0]-7-undecenium chloride, 8-benzyl-1,8-diazabicyclo[5.4.0]-7-undecenium chloride, 8-benzyl-1,8-diazabicyclo[5.4.0]-7-undecenium hydroxide, 8-phenethyl-1,8-diazabicyclo[5.4.0]-7-undecenium chloride, and 8-(3-phenylpropyl)-1,8-diazabicyclo[5.4.0]-7-undecenium chloride; and quaternary phosphonium salts, for example, tetrabutylphosphonium chloride, benzyltriphenylphosphonium chloride, benzyltrimethylphosphonium chloride, and benzyltributylphosphonium chloride.
In the present invention, a mixture of mutually soluble components consisting of the onium compound and the polyhydroxy compound or a salt thereof is used. The mixture of mutually soluble components is a solid solution or a melt.
The solid solution can be prepared by a conventional method. The solid solution used in the present invention can be obtained from the onium compound and the polyhydroxy compound or a salt thereof. The respective components may be converted into a homogeneous state, for example, by a conventional method of mixing both components and melting with heating until both components are liquefied to form a homogeneous mixture, or melting both components with heating under stirring, thereby homogeneously mixing them, and cooling the mixture.
The melt is prepared by heating the onium compound and the polyhydroxy compound or a salt thereof, or a solid solution thereof to at least the melting point, thereby converting into a molten state.
The amount of both components constituting the mixture of mutually soluble components is preferably controlled so that the amount of the onium compound is within a range from 5 to 400 parts by weight, particularly from 10 to 100 parts by weight, based on 100 parts by the polyhydroxy compound or a salt thereof.
The ratio of both components constituting the mixture of mutually soluble components may be previously set to a proper ratio. Alternatively, a mixture of two mutually soluble components in a given ratio is previously prepared and the ratio of both components can also be controlled by adding the polyhydroxy compound or a salt thereof when using the mixture of mutually soluble components. Thus, it is made possible to cope with various weight ratios of the onium on compound to the polyhydroxy compound by only one mixture of mutually soluble components.
At least two mixtures of mutually soluble components may be used in combination. For example, a mixture of mutually soluble components consisting of a quaternary ammonium salt and a polyhydroxy compound can be used in combination of a mixture of mutually soluble components consisting of a quaternary phosphonium salt and a polyhydroxy compound.
The solid solution may be used in the form of powders in order to homogeneously mix with a coagulate of the copolymer. The average particle size thereof is preferably within a range from 10 xcexcm to 2,000 xcexcm, more preferably from 10 xcexcm to 1,000 xcexcm, e.g. 20 xcexcm to 300 xcexcm, particularly preferably from 20 xcexcm to 50 xcexcm.
The content of the mixture of mutually soluble components in the composition for vulcanization is usually within a range from 0.1 to 10 parts by weight, preferably from 1 to 5 parts by weight, based on 100 parts by weight of the fluorine-containing elastomeric copolymer.
According to the process of the present invention, a copolymer is coagulated from an aqueous dispersion and the coagulated copolymer is removed from an aqueous medium by a conventional method.
The coagulation operation may be conducted by a conventional method. For example, a proper inorganic or organic acid (for example, hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, formic acid, acetic acid, oxalic acid, etc.) is added to an aqueous dispersion of a copolymer. An inorganic or organic coagulating agent, for example, aluminum sulfate, magnesium chloride, ammonium chloride, potassium alum, polyalkylamine or a salt thereof can also be used. An acid and a coagulant may be used in combination.
In the present invention, a mixture of mutually soluble components is added to the copolymer coagulate, before, during or after the step of drying the copolymer coagulate. Then, the copolymer is sufficiently kneaded with the mixture of mutually soluble components to obtain a fluororubber composition comprising the copolymer and the mixture of mutually soluble components. The step of drying the coagulate while kneading can be conducted in an extruder. The addition of the mixture of mutually soluble components can be conducted by a metering pump in case of the melt, while it can be conducted by a metering feeder in case of the solid solution.
The method of adding the mixture of mutually soluble components to the copolymer coagulate includes the following three embodiments.
In the first embodiment, the mixture of mutually soluble components is added to the coagulate before the step of drying the coagulate. Before the step of drying the coagulate of the copolymer, the mixture of mutually soluble components is continuously fed to the coagulate removed from the aqueous dispersion and the resultant mix is dried while kneading. The mixture of mutually soluble components is preferably a solid solution, particularly preferably a powdery solid solution. After the mixture of mutually soluble components is added to the wetted copolymer thus coagulated immediately before the drying step, the copolymer and the mixture of mutually soluble components can be dried while melt-kneading. During the coagulate of the copolymer is fed in an extruder for the following drying step, the mixture of mutually soluble components (melt or solid solution) can be continuously fed in the extruder. The powdery solid solution can be fed through a hopper of the extruder. When sufficient kneading and drying are conducted in the extruder, a kneader, which follows the extruder, is not employed.
In the second embodiment, the mixture of mutually soluble components is added to the coagulate during the step of drying the coagulate. The mixture of mutually soluble components in a molten state is preferably added to the copolymer in a molten state. While melt-kneading the copolymer during the drying step in the extruder (for example, twin-screw extruder), the mixture of mutually soluble components in a molten state can be added. The mixture of mutually soluble components is preferably added in the state that the volatile content (for example, water, emulsifier, oligomer, etc.) of the coagulate is small (namely, in the state that the drying step has nearly been completed). The mixture of mutually soluble components may be added to the coagulate whose volatile content (especially water content) is at least 0.01% by weight and less than 1.5% by weight, and particularly from 0.05 to 0.5% by weight. The volatile content means a weight ratio of the volatile component to the whole coagulate. After continuously feeding the copolymer coagulate to the drying step, the mixture of mutually soluble components may be continuously fed to the drying step and then continuously dried while kneading. One extruder may comprises a drying zone where the coagulate is dehydrated and dried, and a kneading zone where the dehydrated and dried coagulate is kneaded with the mixture of mutually soluble components. As used herein, the term xe2x80x9cdehydrationxe2x80x9d means that water is squeezed out from the coagulate between screws. The extruder may be provided with a metering feeder (for example, plunger pump) for feeding the mixture of mutually soluble components so that a weight ratio of the copolymer to the mixture of mutually soluble components in the extruder is constant. When sufficient kneading and drying are conducted in the extruder, a kneader, which follows the extruder, is not employed.
In the third embodiment, the mixture of mutually soluble components is added to the coagulate after the completion of drying of the coagulate. The process of the present invention further comprises the kneading step after the step of drying while kneading, and a melt or a solid solution may be added before the kneading step and after the completion of the drying step. The drying step and the kneading step may be conducted by two extruders, and a first extruder in charge of the drying step and a second extruder in charge of the kneading step can be used. The second extruder may be a surface renewal type kneader. Preferably, the first extruder is a twin-screw extruder, while the second extruder may be either of a single-screw extruder and a twin-screw extruder. A more homogeneous copolymer composition can be obtained by arranging a gear pump between the first extruder and the second extruder.
Depending on the preparation method, the aqueous dispersion of the fluorine-containing elastomeric copolymer used in the present invention contains an emulsifier, particularly when an oil-soluble initiator is used. Since the emulsifier is likely to inhibit the action of the quaternary ammonium compound, the emulsifying action of the emulsifier can be previously lowered by adding an acid before the coagulation of the copolymer.
The acid is preferably added in the amount which makes it possible to adjust the pH of the aqueous dispersion to at most 2.
The acid may be either of an inorganic acid and an organic acid, and examples of the former include hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid, while examples of the latter include carboxylic acids such as formic acid, acetic acid and oxalic acid.
Depending on the kind of the compounds used as the initiator, the aqueous dispersion sometimes exhibits acidic properties. In this case, the aqueous dispersion can be neutralized by adding a base before the coagulation of the copolymer.
The base is preferably added in the amount which makes it possible to adjust the pH of the aqueous dispersion within a range from 7 to 9. As the base, an amine, ammonia, an alkali metal hydroxide, and an alkali earth metal hydroxide can be used. Among these bases, NH4OH and Na2CO3 are preferred.
An additive, which is usually added to the fluororubber composition, may be previously added to the aqueous dispersion. Examples of the additive, which may be added to the aqueous dispersion, include fillers and reinforcers, such as carbon black, clay, diatomaceous earth and talc. If necessary, a plasticizer and a colorant can also be added.